page_title: Secondary surveillance radar
section_title: Mode A
hierarchical_section_title: Secondary surveillance radar / Deficiencies / Mode A
caption_attribution_description: English: This is a widely used diagram associated with Secondary Surveillance Radar
context_page_description: Secondary surveillance radar is a radar system used in air traffic control, that unlike primary radar systems that measure the bearing and distance of targets using the detected reflections of radio signals, relies on targets equipped with a radar transponder, that reply to each interrogation signal by transmitting encoded data such as an identity code, the aircraft's altitude and further information depending on its chosen mode. SSR is based on the military identification friend or foe technology originally developed during World War II, therefore the two systems are still compatible. Monopulse secondary surveillance radar, Mode S, TCAS and ADS-B are similar modern methods of secondary surveillance.
context_section_description: Although 4,096 different identity codes available in a mode A reply may seem enough, once particular codes have been reserved for emergency and other purposes, the number is significantly reduced.  Ideally an aircraft would keep the same code from take-off until landing even when crossing international boundaries, as it is used at the air traffic control centre to display the aircraft's callsign using a process known as code/callsign conversion.  Clearly the same mode A code should not be given to two aircraft at the same time as the controller on the ground could be given the wrong callsign with which to communicate with the aircraft.
caption: Mode A and C reply format
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